1. Field of the Invention
The present invention relates to a processing agent (de-smutting agent) for removing smut formed on the surface of aluminum (as used herein, the term "aluminum" includes both aluminum and aluminum alloys) upon etching.
2. Description of the Prior Art
Etching the surface of aluminum with an acid or alkali aqueous solution is conducted in order to remove oils adhering to the surface of aluminum, an oxidation film which cannot be removed by degreasing, scales, and stains penetrating into the aluminum, to smooth a surface-dressed aluminum, to deluster the surface of the aluminum or to make the surface uniform. Specific examples of acids and alkalis used in the above described acid or alkali solutions include, e.g., acids such as hydrofluoric acid, fluorozirconic acid, phosphoric acid, sulfuric acid, hydrochloric acid, acetic acid, etc., and alkalis such as sodium hydroxide, potassium hydroxide, sodium tertiary phosphate, sodium aluminate, a mixture of sodium silicate and the above-described alkali, a mixture of sodium carbonate and the above-described alkali, etc. In general, etching is effected by processing the surface of aluminum with an aqueous solution of the above-described acid or alkali at a concentration of about 1 to 20 % by weight at a solution temperature of about 50.degree. to 90.degree. C for 10 seconds to 30 minutes.
However, etching processing of an aluminum surface using such as aqueous solution of acid or alkali leads to the formation of areas of a black, insoluble residue, or smut, on the surface. When a plating film or a coating film is formed on the smut-containing surface of aluminum, only a poor close adherence results. Therefore, smut-removal is always conducted.
As the processing agent for removing the smut, a highly concentrated aqueous solution of phosphoric acid, nitric acid, sulfuric acid, chromic acid and a mixture thereof are known.
However, the use of such highly concentrated acidic aqueous solution requires a large amount of alkali in order to neutralize and discharge the acidic aqueous solution. In particular, where chromic acid is used, the discharged water must be completely processed since hexavalent chromium ion affects the human body. Furthermore, the use of nitric acid or sulfuric acid entails the defects that a bad-smelling gas is generated. Also, from the standpoint of the production steps, these conventional methods have the defect that the rate of smut removal is slow.